Colicins Ia and Ib are structurally related protein toxins produced by E. coli strains harboring the pCo1Ia and pCO1Ib plasmids. These toxins are active against certain E. coli strains which they kill by forming aqueous channels in the bacterial cytoplasmic membrane. Strains which produce colicin Ia are insensitive to this colicin, but sensitive to colicin Ib, and vice versa. This property, called immunity, is determined by the same plasmid which encodes the colicin. This proposal extends our previous work on the Ia/Ib system and focuses on three distinct areas: (a) a determination of the amino acid sequence of these toxins, (b) a delineation of the mechanism of immunity, and (c) elucidation of the mechanism whereby expression of the gene which encodes the colicin Ia/Ib receptor, a component of the outer membrane of sensitive cells, is regulated by iron. The primary sequence of both colicins will be deduced from the nucleotide sequence of their cloned genes. For studies on immunity, we will purify both the pCo1Ia- and pCo1Ib-determined immunity proteins. The ability of each to form a complex with pure colicins will be ascertained. Interaction of homologous, but not heterologous pairs, would support our hypothesis that immunity results from specific recognition between homologous pairs with subsequent neutralization of colicin activity. To study regulation of receptor synthesis, regulatory mutants will be isolated and characterized. Mutant and wild type genes will be cloned and sequenced. This approach will allow identification of genetic elements controlling expression of the receptor gene. Since the colicin Ia/Ib receptor may have a physiological function in iron uptake, an understanding of the way its production is regulated by iron may provide insights into the regulation of iron uptake.